Method of forming a diffused metal coded steel product

ABSTRACT

A CODED PRODUCT CONSISTING OF A STEEL SUBSTRATE, AN IDENTIFICATION METAL DIFFUSED ONTO THE SURFACE OF THE SUBSTRATE. AND IN SOME CASES A PROTECTIVE OVERLAY.

Int. Cl. C23c 3/04 US. Cl. 1171 4 Claims ABSTRACT OF THE DISCLOSURE A coded product consisting of a steel substrate, an identification metal diffused onto the surface of the substrate, and in some cases a protective overlay.

BACKGROUND OF THE INVENTION Our invention relates generally to a metallic product which has been treated to make it identifiable at any stage of manufacture. More specifically, our invention relates to a product having a steel substrate which is so treated.

Consumers of steel sheet often require the steel mill to place an identifying mark on the sheet to enable the consumer to determine which mill has supplied it with a particular batch of steel. This is usually done by stamping or etching the surface of the sheet with an identifying mark. In some cases, where a smooth unblemished surface is required, these methods are not feasible. Consumers of tin and chromium plated steel are especially insistent on being able to identify the mill supplying the material while at the same time requiring an unblemished surface.

It is thus an object of our invention to provide a means of identifying the supplier of steel sheet.

Further objects of our invention are:

(1) To provide an identifiable tin or chromium plated steel product;

(2) To provide a means of identifying a steel product Without requiring deplating;

(3) To provide an identifiable plated or coated product having a metallic substrate; and

(4) To provide an identifiable metallic product.

SUMMARY OF THE INVENTION The steel substrate to be coded is cleaned in any conventional manner. We prefer to use steel which has been electrolytically cleaned in an alkaline solution. The cleaned substrate is submerged in a bath of the coding compound and dried. The dried substrate, having a thin film of the coding compound on its outer surface, is then heated to a temperature high enough to degrade the coding compound and to diffuse the metallic cation of the coding compound onto the surface of the substrate. Finally, if a coating is desired, the coded substrate is plated or coated with the desired coating.

Our process thus provides an easy method of coding a steel substrate requiring only one inexpensive step and produces a product with an unblemished surface, identifiable without deplating of the coating, if a coating is present.

DETAILED DESCRIPTION The coding compound should be a metallic salt whose metallic cation is not present in the steel substrate, or which is present in only a minor quantity, and which does not detrimentally affect the surface of the substrate, and whose anion degrades at or below the annealing temperature without producing a residue which would remain on the steel. We prefer to use compounds whose nited States Patent O 3,827,903 Patented Aug. 6, 1974 anion degrades at below 1000 F. It is preferable to use compounds which are readily water soluble and to use water as the solvent, as the solvent is not recovered from the drying step. Other solvents may however be used. We have determined that nickel, cobalt, magnesium, aluminum, calcium, zinc, copper, lead and cadmium are suitable metallic cations. When the product is to be used to make containers for food, we have found nickel, cobalt, magnesium and copper most suitable. Suitable anionic species include oxalates, formates, malonates, acetates and citrates. When water is used as the solvent for the coating compound, we have found the formates and acetates of nickel and cobalt to be especially suitable.

The coding compound may be applied by spraying, dipping, brushing or any other suitable and convenient means. Either the entirety or merely a portion of the substrate may be coated. The coded material is allowed to dry, to prevent solvent from entering the furnace, and is then annealed according to conventional practice.

The solutions of the coding compound are preferably saturated with the coding compound. Where it is desired to obtain higher concentrations of coding compound on the steel substrate, a slurry of the coding compound may be used. Low carbon steel such as is conventionally used as a substrate for plating with tin or chromium is the preferred substrate. The entrained solids picked up by the substrate as it moves through the coding bath remain on the substrate and increase the final concentration. Where it is desirable to obtain lower levels of the coding metal in the substrate, more dilute coding solutions may be used. This procedure allows a wide variation in the concentration of tracer metal in the substrate after annealing. A range of from about 20 milligrams of coding metal per square foot of coded product to about 0.5 milligram per square foot-the lowest level that can be detected by conventional analytical means--is possible using our process. We prefer to use solutions or slurries of a concentration such that the final coding metal concentration is from about 1 to about 5 milligrams per square foot of coded product. To obtain final coding metal concentrations in the above ranges, the dried metallic salt should be present on the surface of the substrate in a concentration of from about 2.5 mg./ft. to about mg./ft. In some instances, it is desirable to cold reduce the coded substrate to meet gage and strength requirements. In such cases, the concentration of the dried metallic salt should be at the higher portion of the stated range. The temperature of the bath is not critical. It is preferred to use the bath at room temperature to avoid the necessity of providing heating or cooling apparatus.

The substrate, coated with the coding compound, is heated to decompose the anion of the coding compound, which then escapes into the air, and to diffuse the cation of the coding compound onto the surface of the substrate. This degradation and diffusion step is preferably accomplished by the standard annealing operation for the steelsubstrate. However, it may also be accomplished by heating in a furnace to a temperature above the decomposition temperature of the coding compound and preferably above 1000 F.

As regards the overlay, we do not intend to be limited to tin and chrominum, the two most common metals plated onto the steel substrate. Other metals may be used. Further, our invention is not limited to metallic overlays. We have found that organic coatings, and lacquer in particular, may also be used with our identification system.

Our process is most beneficially used where the tracer metal can be analyzed without the necessity of deplating or decoating the substrate. X-ray fluorescence is a means of analysis especially suited for use with our system and may be used in instances where the plating or coating on the substrate is not above about mg./ sq. ft. in thickness. Where this thickness is exceeded, deplating or decoating is necessary as a preliminary step to analysis either by X-ray fluorescence or by conventional wet methods. Nor, in its simplest aspect, is our process limited to plated or coated materials, although we anticipate it to have its greatest use in this portion of the art. It is also possible to use our system to identify the source of manufacture of uncoated or unplated material, by following the steps of our process as outlined above, but with the deletion of the plating step.

We further contemplate coding steel substrates with different concentrations of the same tracer metal, thus identifying the manufacturer by the quantity of tracer metal present per unit area.

The tracer metal may already be present in the substrate, provided it is present in such small amounts that the addition of the tracer metal gives an analytically detectable increase in concentration.

Our invention is further illustrated by the following examples, which however, are not intended to be limiting.

EXAMPLE 1 A clean sheet of black plate (cold-reduced low carbon steel sheet) was dipped in a saturated aqueous solution of cobalt formate held at 78 F. This solution contained 13.4 grams of cobalt per liter. The sheet was removed from the bath when it was completely wetted with the coding solution and placed on a drying rack to dry in the air. The dry coated material was annealed at 1200 F. for 30 seconds in an annealing furnace under a protective atmosphere containing about 6% hydrogen and 94% nitrogen. The annealed product was cooled and transferred to the plating operation. After cleaning the surface using an alkaline detergent solution composed of caustic soda, sodium orthosilicate and trisodium phosphate, the black plate was rinsed in water and subjected to electrolytic pickling in a 5 weight percent H 80 solution and a current density was 100 amperes/sq. ft. after which it was fed to a chromium plating bath. The chromium plated black plate was then analyzed using X-ray fluorescence and was found to contain approximately 14 milligrams/ sq. ft. of cobalt.

EXAMPLE 2 Procedure as in Example 1, but using a saturated aqueous solution of nickel formate containing 13.1 grams of nickel per liter. The final product contained approximately 17 milligrams/ sq. ft. of nickel.

We claim:

1. A non-deleterious process for coding a steel substrate,

which comprises:

(a) contacting said substrate with a solution or slurry of a salt selected from the group consisting of the oxalates, formates, citrates, acetates and malonates of cations selected from the group consisting of nickel, cobalt, magnesium, zinc, copper, lead and cadmium, to coat said substrate with an adherent film of sufficient thickness, wherein the subsequent thermal decomposition of said salt and diffusion of said cation into the surface of said substrate will result in a bonded metal layer with a thickness of from about 0.5 to about 20 milligrams of said cation per square foot of substrate surface,

(b) heating said coated substrate to a temperature above the decomposition temperature of said salt, for a time suflicient to degrade the respective anion and diffuse said cation into the surface of said substrate.

2. The process of claim 1, wherein the thickness of said bonded layer is not greater than about 5 milligrams per square foot.

3. The process of claim 2, wherein said heating is achieved during the annealing of said substrate.

4. The process of claim 2, wherein said bonded metal layer is not continuous and is only present on a portion of said steel substrate.

References Cited UNITED STATES PATENTS 2,101,950 12/1937 McGohan 117 R X 2,748,033 5/1956 Gentry et al. 117 46 CA X 1,405,167 1/1922 Shoemaker 117-71 M 1,922,387 8/1933 Miiller 117 46 CA 3,069,765 12/1962 Simpelaar 117 130 R X 3,078,555 2/1963 McFarland 117 46 CA X 3,223,523 12/1965 Adler 117-130RX 3,468,724 9/1969 Reinhold 117130RX FOREIGN PATENTS 562,046 6/1944 Great Britain 117 130R 551,869 1/1958 Canada 117-71 M 598,653 2/1948 Great Britain 117 46 CA ALFRED L. LEAVITI, Primary Examiner I. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.

29183.5, 117-37 R, 46 CA, 71 M, 130 R; 204 38 B 

